The invention relates to diffusion processes for doping semiconductor substrates with phosphorus.
Together with arsenic, phosphorus is the most frequently used dopant for generating semiconductor zones of the n-type. There exist four main processes for phosphorus diffusions. The most frequently carried out is in the "open" tube, i.e., in a diffusion tube which at one end has a sufficiently large opening that a charge with semiconductor wafers can be inserted and removed again, and which at the other end is equipped with at least one gas inlet.
The phosphorus source is principally POCl.sub.3 which is passed over the semiconductor wafers in a carrier of oxygen-nitrogen. If the semiconductor material is silicon, a phosphorus silicate glass is formed on the semiconductor wafers; and the phosphorus then diffuses from the glass into the semiconductor material.
Less frequently, the "open" diffusion uses phosphorus hydride as the source. This compound is highly poisonous.
It is also known to provide on the semiconductor wafers, prior to the insertion into the open tube, a phosphorus-doped oxide pyrolitically or through spin-on of a solution containing a phosphorus compound and silicic acid, said oxide then serving as phosphorus source.
Phosphorus doping may also be done by means of a capsule diffusion, using Phosphorus pentoxide as a source. Due to its highly hygroscopic effect, it is difficult to handle. The capsule diffusion is much more expensive than the "open" diffusion.
Another disadvantage of these known processes is that the entire surface of the semiconductor substrate to be doped is exposed. To obtain a selective doping it is necessary to grow thick layers, e.g. of oxide, for masking; this requires long oxidation processes. But even if these thick oxide layers are grown, there is no guarantee of complete masking because these layers may still have pores which locally may strongly reduce the oxide layer's thickness. Furthermore, in the known "open" phosphorus diffusion processes it is not easy to control the quantity of the phosphorus offered to the semiconductor wafers. Moreover, it is practically impossible with the known phosphorus diffusion processes to generate reproduceably flat, high-resistive (R.sub.s .gtoreq.1200.OMEGA./.quadrature.), n-doped surface layers with fixed characteristics in the semiconductor material. However, such layers are technically very interesting, and it is quite desirable to find feasible processes for their production.
Methods of making such flat, high-resistive n-doped semiconductor structures can for instance be used in the making of n-channel FET's, n-type semiconductor resistors, n-contact diffusions, flat, high-resistive, n-doped layers (n-skin), particularly for setting the necessary threshold voltage of FET's, and in the production of solar cells where it is important to dope large parts of the surface of a semiconductor substrate to form a p/n junction.
The IBM Technical Disclosure Bulletin, Vol. 18, No. 1, June 1975, page 155, teaches a process where a deposition is obtained by dipping semiconductor material into an arsenic dopant. The deposition takes place electrochemically, which is possible with arsenic owing to its metallic properties, but not with phosphorus. This process is not suitable for the reproduceable production of highly resistive doped layers; and it is necessary to use highly poisonous chemical substances.
In RCA Review, June 1970, page 207 ff, and in Solid State Technology, January 1972, page 34, W. Kern described the undesired effect that phosphorus originating inter alia of PO.sub.4.sup.3 ions is adsorbed at semiconductor surfaces, and that when heated to high temperatures this phosphorus diffuses into the semiconductor material, thereby causing undesired dopings. However, this effect results in depositions of only 10.sup.12 phosphorus atoms per cm.sup.2. These are too low for producing technically useable, n-doped layers in p-doped semiconductor material.
Kern also points out that with decreasing pH-value of the solution containing the phosphorus ions the deposition is reduced.